There are publicly known techniques in which an oxygen sensor that detects the oxygen concentration in exhaust gases is provided downstream of a sensor provided in an exhaust system of an internal combustion engine and the air/fuel ratio upstream of the catalyst of the internal combustion engine is forcedly set to a rich condition or a lean condition on the basis of a detected value of this oxygen sensor, thereby to evaluate the deterioration condition of the catalyst (refer to, for example, the Japanese Patent Laid-Open No. H6-129285).
In this conventional apparatus for evaluating the deterioration condition of a catalyst of an internal combustion engine, after the oxygen sensor outputs a detected value of a lean condition or a rich condition, the air/fuel ratio control is reversed so that a target air/fuel ratio becomes a rich condition or a lean condition after a lapse of a predetermined time.